"Is there some type of test (e.g. CCR5 expression) that we could run in advance?"

We don't need to add this level of complexity given what we know from past trials for CV19 (how they decided on the patient population for the Brazil Trials). But since you are lazily attempting to give a reason for bringing the morally corrupt greedy distasteful evil doctor into this conversation, the answer to your question is that the Company already does such tests for their cancer trials.